Papers by David J . X . Gonzalez
Environmental Research Letters
Journal of Exposure Science & Environmental Epidemiology
Background The presence of active or inactive (i.e., postproduction) oil and gas wells in neighbo... more Background The presence of active or inactive (i.e., postproduction) oil and gas wells in neighborhoods may contribute to ongoing pollution. Racially discriminatory neighborhood security maps developed by the Home-Owners Loan Corporation (HOLC) in the 1930s may contribute to environmental exposure disparities. Objective To determine whether receiving worse HOLC grades was associated with exposure to more oil and gas wells. Methods We assessed exposure to oil and gas wells among HOLC-graded neighborhoods in 33 cities from 13 states where urban oil and gas wells were drilled and operated. Among the 17 cities for which 1940 census data were available, we used propensity score restriction and matching to compare well exposure neighborhoods that were similar on observed 1940 sociodemographic characteristics but that received different grades. Results Across all included cities, redlined D-graded neighborhoods had 12.2 ± 27.2 wells km−2, nearly twice the density in neighborhoods graded A ...
Science of The Total Environment, 2021
Background. Prior studies have found that residential proximity to upstream oil and gas productio... more Background. Prior studies have found that residential proximity to upstream oil and gas production is associated with increased risk of adverse health outcomes. Emissions of ambient air pollutants from oil and gas wells in the preproduction and production stages have been proposed as conferring risk of adverse health effects, but the extent of air pollutant emissions and resulting nearby pollution concentrations from wells is not clear. Objectives. We examined the effects of upstream oil and gas preproduction (count of drilling sites) and production (total volume of oil and gas) activities on concentrations of five ambient air pollutants in California. Methods. We obtained data on approximately 1 million daily observations from 314 monitors in the EPA Air Quality System, 2006-2019, including daily concentrations of five routinely monitored ambient air pollutants: PM2.5, CO, NO2, O3, and VOCs. We obtained data on preproduction and production operations from Enverus and the California Geographic Energy Management Division (CalGEM) for all wells in the state. For each monitor and each day, we assessed exposure to upwind preproduction wells and total oil and gas production volume within 10 km. We used a panel regression approach in the analysis and fit adjusted fixed effects linear regression models for each pollutant, controlling for geographic, seasonal, temporal, and meteorological factors. Results. We observed higher concentrations of PM2.5 and CO at monitors within 3 km of preproduction wells, NO2 at monitors at 1-2 km, and O3 at 2-4 km from the wells. Monitors with proximity to increased production volume observed higher concentrations of PM2.5, NO2, and VOCs within 1 km and higher O3 concentrations at 1-2 km. Results were robust to sensitivity analyses. Conclusion. Adjusting for geographic, meteorological, seasonal, and time-trending factors, we observed higher concentrations of ambient air pollutants at air quality monitors in proximity to preproduction wells within 4 km and producing wells within 2 km.
Climate Risk Management, 2021
Environmental Epidemiology, 2020
The oil and gas wells data are publicly available from the California Geologic Energy Management ... more The oil and gas wells data are publicly available from the California Geologic Energy Management Division (CalGEM) through the Geographical Information System (GIS) mapping service. We obtained additional data on the start and end dates for production at each well site from Enverus, a private data aggregation service. The births data are publicly available from the Office of Statewide Health Planning and Development (OSHPD). The data are not available for replication because specific approvals from OSHPD and the California Committee for the Protection of Human Subjects must be obtained to access them.
Environmental Research, 2019
Recent studies from the Madre de Dios region of Peru report elevated mercury exposure among the g... more Recent studies from the Madre de Dios region of Peru report elevated mercury exposure among the general population. Our objective was to assess mercury exposure, risk factors, risk perceptions, and knowledge among women of childbearing age, a population of concern due to risks to infant and fetal health. We collected hair samples and administered a survey among 200 women aged 18-49 years residing in Madre de Dios. Hair total mercury (THg) concentrations were analyzed in accordance with EPA Method 7473. Associations between exposures and potential risk factors were analyzed using generalized linear mixed models. Geometric mean hair THg concentration (± standard deviation) for all participants was 1.62 ± 2.54 μg/g (range: 0.01-30.12 μg/g), and 67 participants (33.5%) had concentrations above the 2.2 μg/g reference dose established by the World Health Organization (WHO). Higher exposure was significantly associated with higher fish consumption (p = 0.03) and less mercury knowledge (p < 0.0001), but not with age or educational attainment. Most participants (86.2%) expressed concern about mercury contamination, but few (26.2%) had knowledge of the health risks associated with exposure. Many residents consumed fish species previously found to have low mercury concentrations, but more than 25% of participants reported consuming fish species with high mercury concentrations. We did not observe cross-sectional differences in fish intake among women who were pregnant or breastfeeding at the time of data collection.
Global Ecology and Biogeography, 2015
Aim Terrestrial ecosystems sequester about 25% of anthropogenic CO2 emissions annually; however, ... more Aim Terrestrial ecosystems sequester about 25% of anthropogenic CO2 emissions annually; however, nitrogen (N) and phosphorus (P) limitation of plant productivity and microbial functioning could curtail this key ecosystem service in the future. Our aim is to address variations in nutrient resupply during decomposition-especially whether the N:P ratio of nutrient recycling via mineralization varies within and across diverse forest biomes. Location Global forest ecosystems. Methods We compiled data on in situ litter decomposition experiments (leaf, wood and root) from the primary literature to examine the relationships between net N and P mineralization across temperate versus tropical forests worldwide. We define net nutrient mineralization ratios as the average N:P released from decomposing substrates at a given ecosystem site. Results We show that net N and P mineralization are strongly correlated within biomes, suggesting strong coupling between N and P recycling in forest ecosystems. The net N:P of leaf-litter mineralization is higher in tropical forests than in temperate forests, consistent with latitudinal patterns in foliar and leaf-litter N:P. At the global scale, the N:P of net mineralization tracks, but tends to be lower than that of litter N:P, pointing to preferential P (versus N) mineralization in forest ecosystems. Main conclusions Our results do not support the view that there is a single, globally consistent mineralization N:P ratio. Instead, our results show that the N:P of net mineralization can be predicted by the N:P of litter, offering a method for incorporating P into global-scale models of carbon-nutrient-climate interactions. In addition, these results imply that P is scarce relative to microbial decomposer demands in tropical forests, whereas N and P may be more co-limiting when compared with microbial biomass in the temperate zone.
Journal of Ecology, 2015
1. Environmental change can affect species directly by altering their physical environment and in... more 1. Environmental change can affect species directly by altering their physical environment and indirectly by altering the abundance of interacting species. A key challenge at the interface of community ecology and conservation biology is to predict how direct and indirect effects combine to influence response in a changing environment. In particular, little is known about how direct and indirect effects on biodiversity develop over time or their potential to influence ecosystem function. 2. We studied how nitrogen (N), winter precipitation (snow) and warming influenced diversity and ecosystem function over 6 years in alpine tundra. We used path analyses to partition direct effects of environmental manipulations from indirect effects due to changes in the abundance of two dominant plants. We hypothesize that (i) indirect effects will develop more slowly but will become stronger than direct effects over time and (ii) after 6 years, indirect effects will more strongly influence diversity while direct effects will influence ecosystem function. 3. Indirect effects of N on diversity were consistently stronger than direct effects and actually developed quickly, prior to direct effects. Direct effects of snow on diversity were detected in year 2 but then subsequently were reversed, while indirect effects were detected in year 4 and grew stronger over time. Overall in year 6, indirect effects were much stronger than direct effects on diversity. 4. Direct effects predominated for three of four ecosystem functions we measured (productivity, N mineralization, winter N availability). The only indirect effects we found were that N and snow indirectly affected microbial biomass N by influencing Geum abundance. Across all four ecosystem measures, indirect effects were infrequent and weaker than direct effects. 5. Synthesis. Increasing indirect effects on diversity over time indicate that short-term experiments or monitoring of natural systems may underestimate the full magnitude of global change effects on plant communities. Moreover, explicitly accounting for changes in dominant plant abundance may be necessary for forecasting plant community response to environmental change. Conversely, weak indirect effects for ecosystem processes suggest that predicting ecosystem function without knowledge of plant responses to global change may be possible.
Current Environmental Health Reports
Purpose of Review Climate change is causing warming over most parts of the USA and more extreme w... more Purpose of Review Climate change is causing warming over most parts of the USA and more extreme weather events. The health impacts of these changes are not experienced equally. We synthesize the recent evidence that climatic changes linked to global warming are having a disparate impact on the health of people of color, including children. Recent Findings Multiple studies of heat, extreme cold, hurricanes, flooding, and wildfires find evidence that people of color, including Black, Latinx, Native American, Pacific Islander, and Asian communities are at higher risk of climate-related health impacts than Whites, although this is not always the case. Studies of adults have found evidence of racial disparities related to climatic changes with respect to mortality, respiratory and cardiovascular disease, mental health, and heat-related illness. Children are particularly vulnerable to the health impacts of climate change, and infants and children of color have experienced adverse perinata...
Climate Risk Management, 2021
The health impacts of wildfire smoke are an important and growing global issue, as extreme wildfi... more The health impacts of wildfire smoke are an important and growing global issue, as extreme wildfire events are expected to increase in frequency and intensity throughout this century due to climate change. Research into individual protective health decision-making can elucidate how wildfire smoke exposure contributes to adverse health outcomes and aid in public health interventions to mitigate risks. In this study we investigate the role of psychological factors (threat and efficacy perceptions) and social processes (social norms and social support) in shaping protective behavior in response to wildfire smoke. Through semi-structured interviews of forty-five individuals in Northern California, we explore perceptions of threat and efficacy, social processes, and protective behaviors in response to wildfire smoke events between 2018 and 2020. We found that for many participants sensory experiences and engagement with wildfire smoke information were instrumental in forming perceptions of threat and efficacy. Three themes related to social processes emerged: interpreting information together, protecting vulnerable others, and questioning protective actions. Through these themes we show how social norms and social support interact in complex, non-linear ways to influence threat and efficacy perceptions, and directly affect protective health behavior. Finally, we propose a conceptual framework of wildfire smoke protective behavior. This study contributes to a growing body of knowledge within the disaster risk and protective health literatures related to wildfire smoke response. Our findings demonstrate how the study of psychological factors and social processes during natural hazards, like wildfire smoke events, is essential to understanding individual protective health decision-making pathways and ultimately, to developing a more comprehensive view of how individual actions affect exposure.
Environ. Res., 2019
Recent studies from the Madre de Dios region of Peru report elevated mercury exposure among the g... more Recent studies from the Madre de Dios region of Peru report elevated mercury exposure among the general population. Our objective was to assess mercury exposure, risk factors, risk perceptions, and knowledge among women of childbearing age, a population of concern due to risks to infant and fetal health. We collected hair samples and administered a survey among 200 women aged 18-49 years residing in Madre de Dios. Hair total mercury (THg) concentrations were analyzed in accordance with EPA Method 7473. Associations between exposures and potential risk factors were analyzed using generalized linear mixed models. Geometric mean hair THg concentration (± standard deviation) for all participants was 1.62 ± 2.54 μg/g (range: 0.01-30.12 μg/g), and 67 participants (33.5%) had concentrations above the 2.2 μg/g reference dose established by the World Health Organization (WHO). Higher exposure was significantly associated with higher fish consumption (p = 0.03) and less mercury knowledge (p < 0.0001), but not with age or educational attainment. Most participants (86.2%) expressed concern about mercury contamination, but few (26.2%) had knowledge of the health risks associated with exposure. Many residents consumed fish species previously found to have low mercury concentrations, but more than 25% of participants reported consuming fish species with high mercury concentrations. We did not observe cross-sectional differences in fish intake among women who were pregnant or breastfeeding at the time of data collection.
Sci. Total Environ., 2022
Background
Prior studies have found that residential proximity to upstream oil and gas production... more Background
Prior studies have found that residential proximity to upstream oil and gas production is associated with increased risk of adverse health outcomes. Emissions of ambient air pollutants from oil and gas wells in the preproduction and production stages have been proposed as conferring risk of adverse health effects, but the extent of air pollutant emissions and resulting nearby pollution concentrations from wells is not clear.
Objectives
We examined the effects of upstream oil and gas preproduction (count of drilling sites) and production (total volume of oil and gas) activities on concentrations of five ambient air pollutants in California.
Methods
We obtained data on approximately 1 million daily observations from 314 monitors in the EPA Air Quality System, 2006-2019, including daily concentrations of five routinely monitored ambient air pollutants: PM2.5, CO, NO2, O3, and VOCs. We obtained data on preproduction and production operations from Enverus and the California Geographic Energy Management Division (CalGEM) for all wells in the state. For each monitor and each day, we assessed exposure to upwind preproduction wells and total oil and gas production volume within 10 km. We used a panel regression approach in the analysis and fit adjusted fixed effects linear regression models for each pollutant, controlling for geographic, seasonal, temporal, and meteorological factors.
Results
We observed higher concentrations of PM2.5 and CO at monitors within 3 km of preproduction wells, NO2 at monitors at 1-2 km, and O3 at 2-4 km from the wells. Monitors with proximity to increased production volume observed higher concentrations of PM2.5, NO2, and VOCs within 1 km and higher O3 concentrations at 1-2 km. Results were robust to sensitivity analyses.
Conclusion
Adjusting for geographic, meteorological, seasonal, and time-trending factors, we observed higher concentrations of ambient air pollutants at air quality monitors in proximity to preproduction wells within 4 km and producing wells within 2 km.
Environ Epidemiol, 2020
The oil and gas wells data are publicly available from the California Geologic Energy Management ... more The oil and gas wells data are publicly available from the California Geologic Energy Management Division (CalGEM) through the Geographical Information System (GIS) mapping service. We obtained additional data on the start and end dates for production at each well site from Enverus, a private data aggregation service. The births data are publicly available from the Office of Statewide Health Planning and Development (OSHPD). The data are not available for replication because specific approvals from OSHPD and the California Committee for the Protection of Human Subjects must be obtained to access them.
Aim
Terrestrial ecosystems sequester about 25% of anthropogenic CO2 emissions annually; however, ... more Aim
Terrestrial ecosystems sequester about 25% of anthropogenic CO2 emissions annually; however, nitrogen (N) and phosphorus (P) limitation of plant productivity and microbial functioning could curtail this key ecosystem service in the future. Our aim is to address variations in nutrient resupply during decomposition – especially whether the N:P ratio of nutrient recycling via mineralization varies within and across diverse forest biomes.
Location
Global forest ecosystems.
Methods
We compiled data on in situ litter decomposition experiments (leaf, wood and root) from the primary literature to examine the relationships between net N and P mineralization across temperate versus tropical forests world-wide. We define net nutrient mineralization ratios as the average N:P released from decomposing substrates at a given ecosystem site.
Results
We show that net N and P mineralization are strongly correlated within biomes, suggesting strong coupling between N and P recycling in forest ecosystems. The net N:P of leaf-litter mineralization is higher in tropical forests than in temperate forests, consistent with latitudinal patterns in foliar and leaf-litter N:P. At the global scale, the N:P of net mineralization tracks, but tends to be lower than that of litter N:P, pointing to preferential P (versus N) mineralization in forest ecosystems.
Main conclusions
Our results do not support the view that there is a single, globally consistent mineralization N:P ratio. Instead, our results show that the N:P of net mineralization can be predicted by the N:P of litter, offering a method for incorporating P into global-scale models of carbon–nutrient–climate interactions. In addition, these results imply that P is scarce relative to microbial decomposer demands in tropical forests, whereas N and P may be more co-limiting when compared with microbial biomass in the temperate zone.
Environmental change can affect species directly by altering their physical environment and indir... more Environmental change can affect species directly by altering their physical environment and indirectly by altering the abundance of interacting species. A key challenge at the interface of community ecology and conservation biology is to predict how direct and indirect effects combine to influence response in a changing environment. In particular, little is known about how direct and indirect effects on biodiversity develop over time or their potential to influence ecosystem function.
We studied how nitrogen (N), winter precipitation (snow) and warming influenced diversity and ecosystem function over 6 years in alpine tundra. We used path analyses to partition direct effects of environmental manipulations from indirect effects due to changes in the abundance of two dominant plants. We hypothesize that (i) indirect effects will develop more slowly but will become stronger than direct effects over time and (ii) after 6 years, indirect effects will more strongly influence diversity while direct effects will influence ecosystem function.
Indirect effects of N on diversity were consistently stronger than direct effects and actually developed quickly, prior to direct effects. Direct effects of snow on diversity were detected in year 2 but then subsequently were reversed, while indirect effects were detected in year 4 and grew stronger over time. Overall in year 6, indirect effects were much stronger than direct effects on diversity.
Direct effects predominated for three of four ecosystem functions we measured (productivity, N mineralization, winter N availability). The only indirect effects we found were that N and snow indirectly affected microbial biomass N by influencing Geum abundance. Across all four ecosystem measures, indirect effects were infrequent and weaker than direct effects.
Synthesis. Increasing indirect effects on diversity over time indicate that short-term experiments or monitoring of natural systems may underestimate the full magnitude of global change effects on plant communities. Moreover, explicitly accounting for changes in dominant plant abundance may be necessary for forecasting plant community response to environmental change. Conversely, weak indirect effects for ecosystem processes suggest that predicting ecosystem function without knowledge of plant responses to global change may be possible.
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Papers by David J . X . Gonzalez
Prior studies have found that residential proximity to upstream oil and gas production is associated with increased risk of adverse health outcomes. Emissions of ambient air pollutants from oil and gas wells in the preproduction and production stages have been proposed as conferring risk of adverse health effects, but the extent of air pollutant emissions and resulting nearby pollution concentrations from wells is not clear.
Objectives
We examined the effects of upstream oil and gas preproduction (count of drilling sites) and production (total volume of oil and gas) activities on concentrations of five ambient air pollutants in California.
Methods
We obtained data on approximately 1 million daily observations from 314 monitors in the EPA Air Quality System, 2006-2019, including daily concentrations of five routinely monitored ambient air pollutants: PM2.5, CO, NO2, O3, and VOCs. We obtained data on preproduction and production operations from Enverus and the California Geographic Energy Management Division (CalGEM) for all wells in the state. For each monitor and each day, we assessed exposure to upwind preproduction wells and total oil and gas production volume within 10 km. We used a panel regression approach in the analysis and fit adjusted fixed effects linear regression models for each pollutant, controlling for geographic, seasonal, temporal, and meteorological factors.
Results
We observed higher concentrations of PM2.5 and CO at monitors within 3 km of preproduction wells, NO2 at monitors at 1-2 km, and O3 at 2-4 km from the wells. Monitors with proximity to increased production volume observed higher concentrations of PM2.5, NO2, and VOCs within 1 km and higher O3 concentrations at 1-2 km. Results were robust to sensitivity analyses.
Conclusion
Adjusting for geographic, meteorological, seasonal, and time-trending factors, we observed higher concentrations of ambient air pollutants at air quality monitors in proximity to preproduction wells within 4 km and producing wells within 2 km.
Terrestrial ecosystems sequester about 25% of anthropogenic CO2 emissions annually; however, nitrogen (N) and phosphorus (P) limitation of plant productivity and microbial functioning could curtail this key ecosystem service in the future. Our aim is to address variations in nutrient resupply during decomposition – especially whether the N:P ratio of nutrient recycling via mineralization varies within and across diverse forest biomes.
Location
Global forest ecosystems.
Methods
We compiled data on in situ litter decomposition experiments (leaf, wood and root) from the primary literature to examine the relationships between net N and P mineralization across temperate versus tropical forests world-wide. We define net nutrient mineralization ratios as the average N:P released from decomposing substrates at a given ecosystem site.
Results
We show that net N and P mineralization are strongly correlated within biomes, suggesting strong coupling between N and P recycling in forest ecosystems. The net N:P of leaf-litter mineralization is higher in tropical forests than in temperate forests, consistent with latitudinal patterns in foliar and leaf-litter N:P. At the global scale, the N:P of net mineralization tracks, but tends to be lower than that of litter N:P, pointing to preferential P (versus N) mineralization in forest ecosystems.
Main conclusions
Our results do not support the view that there is a single, globally consistent mineralization N:P ratio. Instead, our results show that the N:P of net mineralization can be predicted by the N:P of litter, offering a method for incorporating P into global-scale models of carbon–nutrient–climate interactions. In addition, these results imply that P is scarce relative to microbial decomposer demands in tropical forests, whereas N and P may be more co-limiting when compared with microbial biomass in the temperate zone.
We studied how nitrogen (N), winter precipitation (snow) and warming influenced diversity and ecosystem function over 6 years in alpine tundra. We used path analyses to partition direct effects of environmental manipulations from indirect effects due to changes in the abundance of two dominant plants. We hypothesize that (i) indirect effects will develop more slowly but will become stronger than direct effects over time and (ii) after 6 years, indirect effects will more strongly influence diversity while direct effects will influence ecosystem function.
Indirect effects of N on diversity were consistently stronger than direct effects and actually developed quickly, prior to direct effects. Direct effects of snow on diversity were detected in year 2 but then subsequently were reversed, while indirect effects were detected in year 4 and grew stronger over time. Overall in year 6, indirect effects were much stronger than direct effects on diversity.
Direct effects predominated for three of four ecosystem functions we measured (productivity, N mineralization, winter N availability). The only indirect effects we found were that N and snow indirectly affected microbial biomass N by influencing Geum abundance. Across all four ecosystem measures, indirect effects were infrequent and weaker than direct effects.
Synthesis. Increasing indirect effects on diversity over time indicate that short-term experiments or monitoring of natural systems may underestimate the full magnitude of global change effects on plant communities. Moreover, explicitly accounting for changes in dominant plant abundance may be necessary for forecasting plant community response to environmental change. Conversely, weak indirect effects for ecosystem processes suggest that predicting ecosystem function without knowledge of plant responses to global change may be possible.
Prior studies have found that residential proximity to upstream oil and gas production is associated with increased risk of adverse health outcomes. Emissions of ambient air pollutants from oil and gas wells in the preproduction and production stages have been proposed as conferring risk of adverse health effects, but the extent of air pollutant emissions and resulting nearby pollution concentrations from wells is not clear.
Objectives
We examined the effects of upstream oil and gas preproduction (count of drilling sites) and production (total volume of oil and gas) activities on concentrations of five ambient air pollutants in California.
Methods
We obtained data on approximately 1 million daily observations from 314 monitors in the EPA Air Quality System, 2006-2019, including daily concentrations of five routinely monitored ambient air pollutants: PM2.5, CO, NO2, O3, and VOCs. We obtained data on preproduction and production operations from Enverus and the California Geographic Energy Management Division (CalGEM) for all wells in the state. For each monitor and each day, we assessed exposure to upwind preproduction wells and total oil and gas production volume within 10 km. We used a panel regression approach in the analysis and fit adjusted fixed effects linear regression models for each pollutant, controlling for geographic, seasonal, temporal, and meteorological factors.
Results
We observed higher concentrations of PM2.5 and CO at monitors within 3 km of preproduction wells, NO2 at monitors at 1-2 km, and O3 at 2-4 km from the wells. Monitors with proximity to increased production volume observed higher concentrations of PM2.5, NO2, and VOCs within 1 km and higher O3 concentrations at 1-2 km. Results were robust to sensitivity analyses.
Conclusion
Adjusting for geographic, meteorological, seasonal, and time-trending factors, we observed higher concentrations of ambient air pollutants at air quality monitors in proximity to preproduction wells within 4 km and producing wells within 2 km.
Terrestrial ecosystems sequester about 25% of anthropogenic CO2 emissions annually; however, nitrogen (N) and phosphorus (P) limitation of plant productivity and microbial functioning could curtail this key ecosystem service in the future. Our aim is to address variations in nutrient resupply during decomposition – especially whether the N:P ratio of nutrient recycling via mineralization varies within and across diverse forest biomes.
Location
Global forest ecosystems.
Methods
We compiled data on in situ litter decomposition experiments (leaf, wood and root) from the primary literature to examine the relationships between net N and P mineralization across temperate versus tropical forests world-wide. We define net nutrient mineralization ratios as the average N:P released from decomposing substrates at a given ecosystem site.
Results
We show that net N and P mineralization are strongly correlated within biomes, suggesting strong coupling between N and P recycling in forest ecosystems. The net N:P of leaf-litter mineralization is higher in tropical forests than in temperate forests, consistent with latitudinal patterns in foliar and leaf-litter N:P. At the global scale, the N:P of net mineralization tracks, but tends to be lower than that of litter N:P, pointing to preferential P (versus N) mineralization in forest ecosystems.
Main conclusions
Our results do not support the view that there is a single, globally consistent mineralization N:P ratio. Instead, our results show that the N:P of net mineralization can be predicted by the N:P of litter, offering a method for incorporating P into global-scale models of carbon–nutrient–climate interactions. In addition, these results imply that P is scarce relative to microbial decomposer demands in tropical forests, whereas N and P may be more co-limiting when compared with microbial biomass in the temperate zone.
We studied how nitrogen (N), winter precipitation (snow) and warming influenced diversity and ecosystem function over 6 years in alpine tundra. We used path analyses to partition direct effects of environmental manipulations from indirect effects due to changes in the abundance of two dominant plants. We hypothesize that (i) indirect effects will develop more slowly but will become stronger than direct effects over time and (ii) after 6 years, indirect effects will more strongly influence diversity while direct effects will influence ecosystem function.
Indirect effects of N on diversity were consistently stronger than direct effects and actually developed quickly, prior to direct effects. Direct effects of snow on diversity were detected in year 2 but then subsequently were reversed, while indirect effects were detected in year 4 and grew stronger over time. Overall in year 6, indirect effects were much stronger than direct effects on diversity.
Direct effects predominated for three of four ecosystem functions we measured (productivity, N mineralization, winter N availability). The only indirect effects we found were that N and snow indirectly affected microbial biomass N by influencing Geum abundance. Across all four ecosystem measures, indirect effects were infrequent and weaker than direct effects.
Synthesis. Increasing indirect effects on diversity over time indicate that short-term experiments or monitoring of natural systems may underestimate the full magnitude of global change effects on plant communities. Moreover, explicitly accounting for changes in dominant plant abundance may be necessary for forecasting plant community response to environmental change. Conversely, weak indirect effects for ecosystem processes suggest that predicting ecosystem function without knowledge of plant responses to global change may be possible.